As the number of FM channels and station allocations increase, the amount of distortion due to adjacent channel interference also increases. The local channel spacing in the United States is 200 kilohertz between assigned channels. There is usually a dead channel in between local channels so that they do not interfere with each other.
The problem arises when the listener attempts to receive a weak non-local station adjacent to one or in between two relatively strong local stations. Under these conditions, the local stations have information sidebands which, when converted, fall within the passband of the intermediate frequency (IF) amplifier even though the receiver is tuned to the non-local station. Unfortunately, present day stereo detectors are responsive to such unwanted sidebands.
More particularly, the typical IF passband frequency range is plus or minus 100 to 130 kilohertz. The deviation of a station is restricted by law to plus and minus 75 kilohertz giving a total carrier deviation of 150 kilohertz. Unfortunately, the sidebands resulting from the FM modulation process extend out beyond the 75 kilohertz range. Therefore, an adjacent channel which after conversion is centered 200 kilohertz away from the center of the passband of the IF will have sidebands within the IF passband allowing them to be detected by the receiver's FM detection circuitry.
In the stereo demodulator or demultiplexer circuits that are currently used, a 38 kilohertz square wave is employed in the process which separates the left channel from the right channel. The square wave is used because it has no inherent second harmonic content. Unfortunately, the third harmonic of this stereo decoding signal, which occurs at 114 kilohertz, is only reduced in amplitude from the 38 kHz fundamental by approximately 10 db. This means that the amplitude of the demodulated response to undesired sideband signal at 114 kilohertz from the center of the IF passband will only be about 10 db lower than the response to an equal level of stereo information contained on the desired channel. The demodulated response to the undesired sidebands provides unwanted signals which interfere with the desired stereo signals.
One prior art solution to this problem is the use of a very low distortion 38 kilohertz sine wave as a stereo decoding signal. The basic difficulty with this technique is that the sine wave decoding signal does not allow switching techniques to be employed. However, the demodulation must be done in a linear fashion and is undesirably sensitive to the amplitude of the decoding signal itself. Any noise or amplitude modulation contained on the decoding signal then undesirably appears in the output causing reduction in the signal to noise ratio.